Method of sealing anodic aluminum oxide coatings



llnited States Patent O Delaware No Drawing. Filed Mar. 20, 1963, Ser. No. 266,496 5 Claims. (Cl. 20435) This invention relates to the treatment of aluminum and aluminum alloy surfaces to produce thereon coatings possessing an improved reflectivity and resistance to corrosion. More particularly, this invention relates to an improvide procedure for sealing anodized aluminum surfaces whereby an increased resistance to corrosion is obtained and any decrease in reflectivity or polish due to the anodic treatment is minimized.

It is a conventional practice in finishing aluminum surfaces for decorative purposes to first clean and polish the aluminum surface 'by suitable means and thereafter to anodize the polished surface and seal the anodic coating. The polishing stepis generally accomplished by mechanical bufiing of the aluminum surfaces or by treatment of the aluminum in a suitable heated chemical polishing bath. In some instances the chemical treatment may be preceded by mechanical bufiing. The thusly polished aluminum surface is usually anodized to preserve the polished appearance or reflectivity including protecting it from corrosion. However, the anodic coating tends to dull the reflectivity produced by the polishing step more or less in proportion to the thickness of the anodic coating.

Accordingly, for decorative parts such as those used on automobiles where it is desired that the parts have a chrome-like appearance, it is common practice to apply relatively thin anodic coatings over the polished surface in the neighborhood of about 0.1 to 0.2 mil in thickness. Anodic coatings in this thickness range are generally considered by those skilled in the art to not significantly reduce the brightness of the aluminum and to offer adequate resistance to outdoor corrosion to polished automotive aluminum trim parts. However, it has been observed in numerous instances, particularly in highly corrosive environments, that such anodized parts show marked corrosion in service.

It is the basic object of this invention to provide anodic coatings on aluminum which have an increased resistance to corrosion without increasing thethickness of the anodic coatings. It is a more specific object of this invention to provide a method of sealing anodic coatings whereby resistance to corrosion is markedly improved and the luster of the aluminum is not impaired. In general, these and other objects are accomplished by the use of a sealing bath which consists of hot water containing small amounts of disodium 4-dodecylated oxydibenzene sulfonate. Other objects and advantages of the invention will be apparent from the following detailed description thereof.

As generally indicated above, the method of this invention has particular application in the provision of anodic coatings to highly polished aluminum surfaces where it is desirable to provide a high degree of corrosion protection to the polished surfaces without applying an anodic coating of a thickness which would markedly dull the reflectivity of the aluminum.

A wide variety of chemical processes have been developed for polishing aluminum surfaces. One of the more effective and popular processes in commercial use involves first degreasing the aluminum surfaces, for example, in trichloroethylene vapors and then dipping the aluminum for a few seconds in an acid bath heated to about 200 F. consisting essentially of about 80% phosphoric acid, about 4.2% nitric acid and the balance water and small amounts of copper nitrate and other substances ice adapted to improve the effectiveness of the acid bath. Alternatively, the aluminum may be polished by mechanical bufling as is well known in the art. A combination of chemical and mechanical polishing may also be used. An illustration of a mechanical technique for polishing aluminum involves first polishing and bufiing the aluminum surface. Next the aluminum surface is degreased by means of trichloroethylene vapors. The aluminum is then soaked in an alkaline cleaning solution for about three minutes at 150 F. Following this, the panel is rinsed in water and is ready for anodizing.

The anodizing process used for protecting polished aluminum typically involves a sulfuric acid anodizing bath containing from about 10% to 30% sulfuric acid operated at a temperature ranging from about 65 F. to about 115 F. at current densities of from about 4 to amperes per square foot. Anodizing bath temperatures below 65 F. tend to produce undesirable hard dark type coatings and are to be avoided whereas anodizing bath temperatures in excess of F. tend to etch the aluminum rather than anodize it and are also avoided. Preferably, sulfuric acid concentrations of about 12% to 18% are employed. The thickness of the anodic coating will, of course, depend on the time during which the aluminum is subjected to the anodizing treatment.

Following the anodizing treatment the anodized alumi num surfaces are subjected to a sealing treatment in which the pores of the anodically for-med aluminum oxide c0ating are filled or closed. One conventional method of sealing the anodic coating consists of immersing the ano dized part in water heated to a temperature in the order of 200 F. to 212 F. for about 10 to 20 minutes. This results in a conversion of the oxide of the anodic coating to the monohydrate form and a consequent expansion which fills and blocks the minute pores in the anodic coating. The effectiveness of the seal determines to a large extent the corrosion resistance of the anodic coating. Variable conditions which may affect the quality of the seal include the temperature, the pH of the sealing water and its purity, the thickness of the anodic coating and the time of sealing. An excessively low temperature, pH, or time of seal may result in an inadequate seal while an excessively high pH or time of seal may result in an objectionable chalky and smutty surface.

Another method of sealing anodic surfaces involves the use of a hydrolyza'ble salt such as nickel or cobalt acetate in the sealing water. In this type of seal the sealing action proceeds by the precipitation of nickel or cobalt hydroxide in the anodized pores. This type of seal necessitates the use of a wetting agent to prevent continued hydrolysis and the formation of a nickel hydroxide smut over the anodic film. Various proprietary additives have been proposed for addition to a nickel acetate or cobalt acetate scaling bath for eliminating the smut.

The sealing process of this invention involves essentially the use of a distilled or demineralizcd hot water sealing medium to which disodium 4-dodecylated oxydibenzene sulfonate has been added in proportions ranging from about 0.01% to 0.1% by weight. The structural formula for this compound is considered to be SOaNa SOsNa This compound is also known as sodium dodecyl diphenyl oxide disulfonate. The sealing solution is maintained at a temperature of about 180 F. to its boiling point and preferably between 205 F. and 212 F. The pH of the solution is adjusted at about 5 to 7 by means of acetic acid. The sealing procedure involves merely dipping the anodized aluminum surfaces in the sealing solution for at least about five minutes. A sealing timeof about to minutes produces optimum results. The disodium 4- dodecylated oxydibenzene sul-fonate involves an anionactive compound in which the anion is a radical of great ma s n gat ely ch r ed,- It s b li ed tha in h eal g pr es t anion s to som ex ent s r ed n o the por o e c ati Wh n se in pt mum o cent t on i e se l at it app ar at h l cnat comp un s f ct l included i the hyd a and aids vi115 a mo complete blocking of the pores. Concentrations of the sulfonate compound lower than about 0.01% are not effective under practical operating conditions whereas concentrations greater than about 1% increase the rate of adsorption, slow down the sealing process and become detrimental to corrosion resistance.

The corrosion resistance of anodic aluminum coatings sealed in accordance with this invention was determined by the Cass test in which the sealed anodie coating is posed t a c ppe ch r de mod fied ac t ac d a spray. Specifically the spray consisted of a 5% by weight sodium chloride solution containing 1 gram per gallon of the dihydrate of cuprous chloride and adjusted to have a pH of 3.2:1. Identical samples of anodized aluminum w anod zed o have a ano i c a ng of 0.2 mi in thickness by means of an anodizing bath containing 15.0% sulfuric acid operated at 72 F, at a current density of 12 amperes per square foot operated for 15 minutes. One group of about ten samples was sealed for ten minutes in distilled water maintained at about 210 F. Another group was sealed for ten minutes in distilled water containing about 0.05% disodium 4,-dodecylated oxydibenzene sulfonate and about 0.25% sodium acetate as a buffer to maintain the pH between about 6.5 adjusted with acetic acid and held at a temperature of about 210 F. The samples were subjected to the Cass test. This in? volved placing the samples in a cabinet heated to about 120 F. into which the previously mentioned copper chloride modified acetic acid salt spray is sprayed at a rate h a 1 to 1 2 millig am-s ofwatcr are coll cte as a condensate per hour. 11 o instance did samples subjected to the conventional hot water seal Withstand the salt spray for six hours without corrosion. On the other hand, the samples sealed in accordance with the process of this invention withstood the salt spray for sixteen hours without corrosion. a

A substantial number of .other'sirnilar aromatic sulfonate anion-active compounds were tested. Most produced no significant corrosion resistance improvement. A number of the compounds tested produced some improvement. However, no other compound tested compared favorably in effectiveness and reliability. It is believed that the eifectiveness of the disodium 4-dodecylated.

oxydibenzene sulfonate is due to its specific chemical structure and is unique in its behavior in the sealing bath of this invention. Another important aspect of the seal- 4 ing bath of this invention is'that the disodium 4-d0decylated oxydibenzene sulfonate in the hot water sealing bath eliminates the chalky appearance usually encountered in hot water sealing procedures.

While the embodiment of this invention as disclosed herein eonstitptes a preferred form, it is to be understood that other forms may be adopted without departing from the sp i o the inve ti n.

-I claim:

1. A method of sealing an anodically formed aluminum oxide coating comprising the steps of immersing said coating in a sealing solution heated to a temperature of about 205 F. to 212 F. consisting essentially of Water, 0.01% to 0.1% by weight of disodium 4-dodecylated oxydibenzene sulfonate, sufiicient acid to establish a pH of between 5 and 7 and small quantities of a buffer to maintain said pH, and maintaining said coating in said solution for a period of' from about five to about twenty minutes.

2. The method of claim 2 in which said acid is acetic acid and said buffer is sodium acetate.

3. A method of forming decorative aluminum parts comprising the steps of bright polishing the aluminum surfaces, anodically forming an aluminum oxide coating on said surfaces in a thickness range of from aboutlll to about 0.2 mil, and sealing said coating by immersing it for at least five minutes in a sealing solution heated to about 205 F. to 212 F. consisting essentially of water about 0.01% to 0.1% by weight of disodium 4-dodecylated oxydibenzene sulfonate and sufficient acid to establish a pH of about 5 to 7.

4. A method of forming decorative aluminum parts 7 comprising the steps of bright polishing the aluminum surfaces, anodically forming an aluminum oxide coating on said surfaces in a thickness range of about 0.1 to 0.2

'mil, and sealing said coating by immersing it for about five to twenty minutes in a sealing solution heated to about 205 F. to 212 F. consisting of water, about 0.01% to 0.1% by weight of disodium 4-.dodecylated oxydibenzene sulfon'ate, suflioient acid to establish a pH of about 5 to 7 and suflicient huifer to maintain said pH. 7

5. A method of sealing anodically formed aluminum oxide coatings comprising the steps of immersing the anodized aluminum into a solution heated to a temperature of about 180-to 212 F. consisting essentially of Water, 0.01% to 0.1% by weight of disodium 4-dodecyla-ted oxydibenzene sulfonate, and sufiicien-t acid to establish apH of between about 5 and 7, and maintaining said aluminum in said solution for at least about five minutes.

References Cited in the file of this patent UNITED STATES PATENTS 2,814,576 Zickendraht NOV. 26, 1957 2,956,935 Passal Oct. 18, 1960 3,026,255 Riou et al Mar. 20, 1962 

5. A METHOD OF SEALING ANODICALLY FORMED ALUMINUM OXIDE COATINGS COMPRISING THE STEPS OF IMMERSING THE ANODIZED ALUMINUM INTO A SOLUTION HEATED TO A TEMPERATURE OF ABOUT 180* TO 212*F. CONSISTING ESSENTIALLY OF WATER, 0.01% TO 0.1% BY WEIGHT OF DISODIUM 4-DODECYLATED OXYDIBENZENE SULFONATE, AND SUFFICIENT ACID TO ESTABLISH A PH OF BETWEEN ABOUT 5 AND 7, AND MAINTAINING SAID ALUMINUM IN SAID SOLUTION FOR AT LEAST ABOUT FIVE MINUTES. 